In order to develop high-integration semiconductor devices with a thinner film structure, the film's kind and structure has been variously studied, and, for example, new CMOS gate structures have been proposed. In a conventional CMOS gate structure, a gate insulating film is formed on a silicon film, and a thinning of the gate insulating film has been progressed. However, a leakage current increases as the gate insulating film becomes thinner, so that the conventional gate structure has a limitation to the thinning thereof.
As one of the new CMOS gate structures, there is, e.g., a so-called three-dimensional gate structure. In such a structure, it is required to form a gate electrode having a fine and three-dimensional shape. Further, in an aspect of a material, a metal gate electrode is under investigation instead of a conventional polycrystalline silicon electrode, and accordingly, considerably complex processes are required in a next-generation semiconductor device manufacturing process.
As one of the processes, a process for etching a silicon nitride (SiN) film on an underlying silicon oxide (SiO2) film has been studied. The patterned silicon nitride film formed by the etching is used in forming the gate electrode later.
However, in general, in case of etching a film, an etching rate of a substrate surface is not completely uniform, and it is difficult to make the etching rates of a central portion and a peripheral portion identical to each other. Accordingly, the etching is continued after the film is etched to expose an underlying film, which is called an overetching. Generally, in case of the overetching, it is required to ensure a high selectivity of the film to the underlying film, but the selectivity of the film to the underlying film is approximately 7 to 10 at most.
However, in a semiconductor device structure being considered herein, the silicon oxide film as the underlying film is very thin, e.g., 5 nm. In this case, a high selectivity of, e.g., about 20 to 40 is required to perform the overetching. Japanese Patent Laid-open Application No. 2003-229418 discloses a method for performing an etching by using, as an etching gas, a gaseous mixture containing CH3F gas and O2 gas wherein the mixing ratio (O2/CH3F) of the O2 gas to the CH3F gas is 4 to 9, in order to increase the selectivity of the silicon nitride film to the silicon oxide film when etching the silicon nitride film by using the silicon oxide film as a mask. Therefore, in the process for etching the silicon nitride film on the underlying silicon oxide film, by using the gaseous mixture upon the overetching, it is possible to etch the silicon nitride film while suppressing the reduction of the underlying film even though the underlying silicon oxide film is thin.
However, in case of using the aforementioned gaseous mixture, there occur problems as described below. FIG. 10A is a view showing a laminated body before etching the above-described silicon nitride film, and in FIG. 10A, reference numerals 11, 12, 13, 14 and 15 denote a silicon (Si) film, a silicon oxide film, a silicon nitride film, a nitrogen containing silicon oxide (SiON) film serving as a hard mask, a resist film and a resist pattern formed in the resist film 15, respectively. In case of performing a conventional etching on such laminated body, the SiON film 14 is etched by using the resist film 15 as a mask, the resist film 15 is removed by ashing, and an etching is then performed with the gaseous mixture described in Japanese Patent Laid-open Application No. 2003-229418 by using the SiON film 14 as a mask.
However, as described above, if an etching is performed by using a gas having a very high selectivity of the silicon nitride film 13 to the silicon oxide film 12, an etching action to the silicon nitride film 13 is strong. At this time, since the silicon nitride film 13 and the SiON film 14 serving as a hard mask have a similar material composition, the gaseous mixture causes local damage to the SiON film 14. Further, because the SiON film 14 is thin, e.g., 50 nm, and the SiON film 14 is not completely uniform even if it has an in-surface uniformity within a specification, holes 18 are formed in the SiON film 14 as confirmed by an experiment. If there occurs such a phenomenon wherein the holes 18 are formed (hereinafter referred to as “pitting”), a surface of the silicon nitride film 13 is damaged through the holes 18, which affects a next process.
It may be considered to protect the SiON film 14 without ashing the resist film 15 to remain when etching the silicon nitride film 13. However, because the gaseous mixture contains a large amount of oxygen, the resist film 15 is ashed when the silicon nitride film is etched. Therefore, in order to protect the SiON film 14, a film thickness of the resist film 15 has to be large. However, in that case, an etching profile is deteriorated. That is, in order to improve the etching profile, the resist film 15 is required to be thin. Under these circumstances, there is demanded an etching method capable of performing the above-described etching without damaging the silicon nitride film.
Further, Japanese Patent Laid-open Application No. 2000-269220 discloses, as a method for forming a hard mask of silicon nitride, a technology capable of thinning a silicon nitride layer by oxidizing an antireflection film on a silicon nitride layer by using an oxygen plasma generated when a resist mask located on the antireflection film is ashed to thereby use the oxidized antireflection film as a protection layer. However, the technology cannot achieve the object of the present invention.